Apparatus for heat-treating metal parts



DecQ 7, 194s. H, F, STORM 2,455,822

APPARATUS FOR HEAT TREATING METAL PARTS Filed June 7, 1944 FIG. l

INVENTOR HERBERT F. STORM ATTORNEYS Patented Dec. 7, 1948 APPARATUS FR HEAT-TREATING METAL PARTS v Herbert F. storm, wat Ams, wis., signor to Sunbeam Corporation. a corporation of Illinoia Application June 7, 1944, Serial No. $9,105

(Cl. 26S-4) Claims.

l The present invention relates to methods and apparatus for heat treating metal parts and more particularly to improved methods and apparatus for utilizing a' high frequency magnetic field to' heat the inner surface of a hollow ferrous metal part tc a hardening or quenching temperature and for then quenching the heated surface.

It is common practice to utilize high frequency magnetic iields to heat a selected surface zone of a ferrous metal part for the purpose of case' hardening the selected zone without destroying the ductility oi the other portions ofthe part. Usually the induction heating is accomplished by bring an inductcr coil into proximity with the selected surface zone and then energizing the inductor from a high frequency electronic oscillator for a predetermined time interval which is calculated to heat the selected surface zone to.

. the desired quenching or hardening temperature.

As is well known, the magnetic field causes hysteresis and eddy current losses to be manifested as heat in the selected surface zone o? the part, and the extent o1' heating is determined by the intensity oi' the generated magnetic field. This of course means that the rise in temperature which is realized in a selected time interval is dependent upon and changes with variations in the voltage of the current source from which the high frequency energy is derived, i. e., the current source from which anode current is supplied tc the high frequency oscillator tube.

Any heating interval selected on an empiricai or calculated basis cannot take such source voltage variations into account, due to the unpredictability thereoi, with the result that time and heat energy may be wasted when the induction heating is effected on a timed basis. Also, in the use 0I induction heating apparatus to heat the internal surfaces of small metal parts, such, ior example, as rivet dies, nuts, or the like, the space problem becomes serious. Thus, the usual in ternal surface heating inductor is in the form oi a helical coil of Vhollow copper tubing which is adapted to belnserted into the part opening in such manner that the generated magnetic eld is parallel to the axis of the opening, and is con structed of copper tubing oi suillclently large diameter to permit circulation of a liquid coolant ing of bore holes having small diameters. Fur ther, the known prior art arrangements for induction heating and subsequent quenching or v small metal parts usually require either manual handlingA of the parts or the use of relatively complicated apparatus, in .successively eil'ecting the heating and quenching operations.

It is an object oi' the present invention, there fore, to provide improved high frequency indueE tion heating methods and apparatus wherein the above-noted problems are wholly obviated.

therethrough. Such an inductor obviously is not It is another object oi the invention to pro vide improved methods and apparatus for the high frequency induction heating oi magnetic metal parts to a uniform predetermined temper ature regardless oi variations in the voltage ci? the current source from which the high frequency energy is derived.

it is afurther object ci the invention to preevide an improved and fully automatic arrange ment for discontinuing the application of the high frequency magnetic eld to each metal part when the field traversed surface zone oi the part is heated to a predetermined temperature.

.According to a more specic object o?. the ine vention, the sharp change in magnetic perlrse-= ability of the metal part which occurs when. the part is heated tc the desired predetermined terse perature, is utilized to discontinue the applica@ tion of the high frequency field to the part.

It is a still further object oi. the invention to provide an improved arrangement for automatin cally effecting movement of each meta?. part from a heating position to a quenching,- position immediately the field traversed surface zone of the part is heated to the desired predetermined temperature.

in accordance with stili another object oi the invention movement of each part trom its heat= ing position to its quenching position is ically effected in response to the change in pere meability of the part which occurs when the part is heated to the desired quenching or hard enirig temperature.

According to a further object of the invention the apparatus is provided with improved raciti ties for flowing a quenching liquid against the heated zone oi each metai part after it is heated to the desired predetermined temperature, and

attacca the dow of quenching liquid is automatically changed in response to the change in permeability of the part which occurs when the part is heated to the desired quenching or hardening temperature.

It is a further object of the invention to provide exceedingly simple and completely reliable induction heating apparatus for easily and rapidly heating the internal surfaces oi hollow metal parts to a predetermined temperature.

It is still further and more specific object of the invention to provide improved induction heat ing apparatus of the character described, which is particularly adapted for use in heating the internal surfaces of bore holes or the like ci small diameter.

The invention, both as to its organization and method of operation, together with further obiects and advantages thereof will best he andere stood byreference to the specication taken in connection with the accompanying drawings in which:

Fig. l. schematically illustrates improved high frequency induction heating apparatus char acterized by the features of the present invention; and

Fig. 2 is a sectional view taken along the lines 2--2 in Fig. l..

Referring now to the drawings, the improved high frequency induction heating and quenching apparatus there illustrated is well adapted for use in hardening the internal surfaces of small rivet dies without imparing the ductility of the outer die segments. A typical die of this character is indicated in section at til, as being provided with a bore hole lila extending longitudinally thereof. It is of cylindrical form having an outside di ameter ci approximately one inch, is approximately two inches in length, is provided with a central bore hole having a diameter o threesixteenths or an inch, and may be formed of any suitable steel having magnetic properties and the required toughness. ln using this die, a steel. wire segment is inserted into the bore hole ida having a length such that one end thereof projects upwardly from one end of the die. n. powerful blow is then brought to bear against the projecting portion oi the wire segment, with the result that the projecting portion or the wire segment is ilattened to form the rivet head. lincident to this blow, that portion of the wire segment disposed within the bore hole is swelled against the bore hole surfaces, thus causing severe stresses to be developed within' the die in directions extending radially of the bore hole axis. In order that. the die structure may absorb these stresses without undue wear or failure, the inside of the bore hole must be hardened. Since hardness and brittleness are paired characteristics in many steels, it is necessary that the hardening of the internal die surfaces be effected without hardening of the other portions oi the die. The use of known methods ci. case hardening the interna] die surfaces results in a rather complicated and critical procedure. In accordance with one conventional method, the entire die is first heated throughout and hardened throughout, following whlch the die is reheated for tempering, in order to reduce the brittleness in the outer die section. This tempering process requires close attention in order to be successful. For example, the heat must be applied long enough to bring the outer section of the die up to the desired tempering temperature, but must not be applied long enough to destroy the hardness of the internal die surfaces. The present improved apparatus is well adapted for use in hardening the internal die surfaces without in any way disturbing the ductility or rendering brittle the outer die sections.

Brieily considered, this apparatus comprises a rod-like inductor element il in the form of a straight elongated copper tube which is adapted to be snugly received within the bore hole lila of the rivet die it or another hollow metal part; a=

high frequency oscillator indicated generally at it for impressing a high frequency voltage be tween the end portions of the inductor element ii; and a quenching liquid supply system comprising the valved pipes it and l5 for flowing a quench and cooling liquid, such, for example, as water or oil through the inductor element li. The oscillator iii is of the well known Colpitts type, being provided with a three electrode electron discharge tube 2&3 having output electrodes edd and iic coupled through o, coupling condenser Sd to 'a tuned oscillatory circuit 27. This circuit is comprised or" an inductance coil 21a shunted by series tuning condensers 21o and 21o and may, for example, lbc tuned to a resonant frequency of one-half megacycle. Sustained operation of the oscillator is obtained by impressing that portion of the oscillatory voltage which appears lacross the condenser 21e between the input electrodes 26g and 28e of the tube 26 through a coupling condenser 3l. The grid circuit of the tube 28 also includes a biasing network made up of the operating winding of a control relay 40 and a series connected resistor 3l, the two identified elements being connected in series between the input electrodes of the tube 28 through a high frequency lter choke 33 and being lay-passed to ground for high frequency currents by means of a iilter condenser it. .anode current is supplied to the space current path of the tube 2S from a direct current source 86 through the contacts 52 of s. start-stop circuit breaker or relay B0 and a high frequency choke inductance 35. This inductance performs the function of preventing the high frequency voltage developed across the oscillatory circuit 2l from effecting high frequency current dow through the current source 3B. As will be evident from the following explanation, considerable power is required to heat the internal surfaces of the die il) to a quenching temperature and for that reason it is usually not practical to use a battery as the anode current source for the tube 28. It will be understood, therefore, that the. battery indication is used for convenience only and that in practice this source may comprise the usual alternating current voltage step up and rectifying facilities, and hence may be subject to voltage variations in response to changes in the load imposed thereon and also 'vided with a slit 32a extending longitudinally thereof to interrupt the continuity of the conductive path around its circumference. Intermediate the edges defining the slit 32a, the sheath 32 is grounded as indicated at 32d in order to preclude any possibility of a high voltage appearing upon the inductor element H which might be manana dangerous `to users of the apparatus. This sheath is also provided with terminal parts 32h and 32e adjacent the edges of the slit 32a, which parts are respectively connected to the end portions oi the inductor element Il. Thus the terminal part ne is connected to the upper end portion of the inductor element I I through a copper or other nonmagnetic conductive bracket 24. The opposite terminal part 32h is connected to a pivotally mounted work'supporting assembly 53 of paddle shaped contour, the parts oi which are likewise formed oi copper or another non-magnetic conductive metal. This assembly is comprised of pivotally connected jaw members 53a and 33h which are tensioned to close upon the conduit I2 by means of a biasing spring 53e, the members 53a being pivotally mounted upon a pivot rod 5l to permit the assembly to be moved into and out of embracing engagement with the inductor element II. The spring biased engagement of the jaws 53a and r83h with the inductor element II insures a good electrical connection between this element and the parts of the assembly 53. The die supporting arrangement thus provided facilitate placement of the die IB telescopically over l the inductor element II and removal of the die from the inductor element after the heating and quenching operations are completed in the manner explained below.

As indicated above, the die I is slidably movable along the inductor element II from a heating position to a cooling or Vquenching position. The heating position of the die'is illustrated in full lines in Fig. 1 of the drawing, the upper peripheral portion of the die being held within the edges of a .frame member 25 which may be rigidly mounted adjacent the upper end of inductor element I I in any suitable manner. With the die in this position it is obviously gravity biased to slide downward along the inductor element II to the dotted line quenching position wherein it is supported by the member 53. For the purpose of ilowing the quench liquid against the heated internal surfaces of the die when it occupies the quenching position, the inductor element I'I is provided with a number of small wall openings I2 which are spaced circumferentially around the inductor element and extend upward from the supporting surface of the member 53 along the element Ii a distance equal to the length of the die. A holding magnet 23 suitably supported from the bracket 24 and adapted for energization from a current source 39 is utilized to hold the die I0 in its elevated heating position for the duration. of the heating interval. This magnet is arranged to be energized through the contacts of a switch 38 and may be de-energized under the control of the relay 40.

As indicated above, the conduit system comprising the two conduits I4 and I5 is utilized to flow a quenching liquid through the inductor element II for the dual purpose of preventing this inductor element from overheating during energization of the element from the high frequency oscillator I3 and of owing the quenching liquid through the openings I2 against the internally heated surfaces of the die I0 after the die has been moved to its cooling or quenching position. The rate of quenching liquid flow required to prevent the inductor element I'I from overheating is substantially less than that required for effective quenching of the internally heated survfaces of the die after the heating operation is completed. In order, therefore, to increase the rate oi quench liquid now through the `inductor element II coincident with movement oi' the die I0 from its heating position to its quenching position. an electromasnetically controlled valve I1 is connected in parallel with the valved conduit Il. This valve includes an actuating arm I'Ia and is normally biased to its' closed setting by means of a tension spring I3, the opposite ends of which are anchored to the arm Ila and to a bracket piece I3 projecting horizontally fromy the upper end oi' the inductor element II. Under normal conditions and during a die heating operation, the iiow of quenching liquid through the inductor element II is exclusively controlled by the setting of a valve I6 included in the conduit I5, this valve being suitably adjusted to permit an absolute minimum flow of quenching liquid through the inductor element Il consistent with the desired end of preventing this element from over-heating. For the purpose of pivoting the actuating arm I'Ia. thereby to open the valve Il and thus increase the flow of quenching liquid through the inductor element I I, an electromagnetic solenoid 20 is provided. This solenoid is supported at one end of the bracket 2i carried by the conduit I4 and includes a plunger 20a which is connected to the outer end of the arm I'la through a link element 22. The operating winding 20h of this solenoid is arranged for energization from the current source 39 under the control of the relay 40. As pointed out above, operation of the oscillator I3 to energize the inductor element II is under the control of startstop circuit breaker 50. This circuit breaker is arranged for energization from the current source 39 through the contacts of a self-restoring start key or switch 3l.

In considering the operation of the above described apparatus it will be understood that before initiating operation of this oscillator I3, a die is iirst telescoped over the inductor element II and moved to the described heating position. To this end, the supporting assembly 53 is pivoted away from the inductor element II leaving the lower end of this element free to project within the bore hole Illa of the selected die. 'I'he die may now be teles'coped over the inductor element II and elevated to its heating position against the frame 25. In order to retain the die in this position, the switch 38 is closed to prepare a holding circuit for the solenoid 20 and to oomplete a circuit through the contacts -43 of the relay 40 for energizing the winding 23a of the holding magnet 23. With this magnet energized, the die I0 is securely held in its elevated position. Following the described operations the single turn inductor pickup circuit may be completed by reversely pivoting the supporting assembly 53 to re-engage the jaws 53a and 53h with the lower end of the inductor element Il, an electrical connection between the identified parts being insured through the spring biased engagement of the jaws with the circumferential surface of the inductor element. The final preparatory operation is that of opening the valve I6 to initiate the flow o! quenching liquid through the inductor element I I.

In order to initiate operation of the oscillator I3 following the preparatory steps just described, the key 31 is actuated to its closed circuit position to complete an obvious circuit for energizing the circuit breaker 50 from the current source 39. This circuit breaker, in operating, locks up in a circuit which includes the current source '38 and the contacts Si and di.

At its contacts path through the filter choke @El for connecting the positive terminal of the anode current source 56 to the anode 28a of the tube 2G. Incident to the last mentioned switching operation, oper-s ation oi the oscillator i3 is initiated. is pre `yiously mentioned, operation of the oscillator lil is sustained by impressing the generated high frequency voltage appearing across the condenser @le upon the cathode-grid circuit of the tube 26 through the coupling condenser ill. 'The bias voltage appearing between the cathode and grid of this tube is developed `by grid rectification of the current traversing thewinding of the relay d@ andthe resistor Sil.

Due to the magnetic coupling between the tuning inductance Zlio of the oscillator output circuit 2li and the pickup sheath 82, and the high turn ratio between the inductance element Zlio and the single turn inductor pickup circuit, a high frequency current having a large order of magnitude is caused to traverse the inductor element il. Since the inductor pickup circuit is of low impedance and is grounded at the point 12d, all exposed parts thereof are at a low po tential. In this regard it is noted that the connections between the terminal parts 32h and 32o, the supporting bracket 24 and the supporting member 53 are made as short as possible in order tominimizevthe impedance of the inductor pickup circuit.

The high frequency current traversing the inductor element il causes a high frequency mag netic field to be developed circumferentially of this element, i. e. peripherally of the bore hole iba through the die lli. lLn other words, the nur lines of this magnetic field traverse the adjacent internal surface portions of the die lil in planes which are normal to the longitudinal axis of the bore hole. The resulting hysteresis and eddy currents set up in the internal surface portions of the die I cause these portions of the die to be rapidly heated. The eld penetration into the metal is of course very slight at the high frequency used, and hence the heating is strictly limited to the surface segments of the bore hole. By providing a magnetic field of sufficient intensity within the bore hole, the heating eect of the bore hole surface may be carried out with sufficient rapidity to prevent substantial amounts of heat from being conducted away from the internal surface portions of the die as the heat is generated. In fact, by providing a field of sufficiently high intensity, the internal surface areas of the die may be heated to a red temperature without raising the temperature of the outer surface portions of the die more than a few degrees.

It has been found that the magnetic permeability of magnetic steels normally used in the construction of dies and other parts of the character under consideration, decreases sharply when the steel is heated to a predetermined temperature. More specifically, it has been found that the temperature at which this permeability change occurs in a carbon steel part, for example, is at least equal to the temperature which must be imparted to the part in order to effect the desired case hardening of the part when it is subsequently subjected to a quenching operation. This sharp change in the magnetic permeability of those internal surface portions of the die It) which are subjected to the high frequency magnetic field is utilized automatically to arrest the operation of the oscillator i3A when the indicated die portions are heated to the desired quenching temperature. Thus, it will be understood that the load imposedupon the oscillator i3 is deiinitely a function of the heat developed within vthe internal surface portions of the die i0.

When the heat developed in these surface portions is decreased due to the sharp decrease in the permeability of the heated die parts which occurs when these parts are heated to the desired quenching temperature, the load imposed upon the oscillator t3 is sharply reduced. lin response to this change in the oscillator load, the rectified grid current traversing the winding of the relay [it and the resistor 34 is greatly increased. This relay is adjusted to operate in response to the described increase in rectified grid current flow therethrough. Upon operating, it opens its contacts il to del-energize the circuit breaker Eil. At its contacts dt, the relay do de.- energizes the winding 23a of the holding magnet At its contacts t2, the relay d completes an obvious circuit for energizing the operating winding 26h of the solenoid 2o.

The circuit breaker 50, upon restoring, opens its contacts 5l further to interrupt its own locking circuit. At its contacts 52, the circuit breaker 50 interrupts the anode current supply circuit for the tube 26, thereby to arrest the operation of the oscillator i3. When the winding 23a of the holding magnet 23 is de-energized the die l0 is'released for movement along the inductor element i l to the illustrated cooling position, such movement being effected through the influence of gravity pull upon the die I0 and being arrested when the lower face of the die contacts the upper surface of the member 53. When the solenoid 2d attracts its plunger 20a inresponse to the energization of its winding 20h, it pivots the actuator arm l1 against the bias ofthe spring I8 to open the valve il, thereby to increase the flow of quenching liquid through the inductor element il. Also incident to the pivotal movement of the valve actuating arm lla, the contacts Hb are closed to complete a circuit through the switch contacts 38h for holding the winding 20h energized independently of the relay Ml.

From the above explanation it will be understood that incident to the operation of the relay 40, the operation of oscillator I3 is arrested, the die l0 is moved from its heating position to its cooling position, and the ow of quenching liquid through the inductor element Il is increased to enhance the quenching of the heated internal surface, portions of the die. In this regard it will be understood that the quenching liquid in flowing through the wall openings I2 of the inductor il impinges upon the heated internal surfaces of the die I0 to rapidly cool these surfaces and thus effect the desired case hardening thereof. Incident to the described interruption of the anode current supply circuit for the tube 26, grid current flow through the winding 40 is obviously terminated causing this relay to release. Upon restoring, the relay I0 recloses its contacts Il to reprepare the locking circuit for the circuit breaker 50. At its contacts 43, the relay 40 recompletes the circuit for energizing the holding magnet winding 23a, this operation being without effect since prior to its occurrence the die Ill is removed from the sphere of attractive influence of the magnet 23. At its contacts 42, the relay 40 interrupts the operating circuit for the solenoid winding 20h, but the latter circuit remains energized through the contacts 20h and the switch contacts 38h.

After the die Il has been quenched in the inanner explained above; the switch 3l may be returned to its normal position, wherein the contacts #la and llb are open. Incident to this operation, the solenoid 2li is de-energized, permitting the valve l1 to be closed under the influence of the spring i8. As the valve actuator arm I'Ia is operated to close the valve l1, the contacts lib are opened, thereby to prevent premature energization of the solenoid 20 when the switch 38 is subsequently closed. Incident to the opening of the contacts Ila, the winding 23a of the holding magnet 2i is obviously de-energized. After the described operations are completed, the valve' Il may be closed to arrest the fiowvof quenching liquid through the inductor element li, and the die I0 removed from its -cooling position by pivoting the support assembly 53 away from the Inductor element and then sliding the die downward until it is free of the lnductor element. A second die may now be telescoped over the inductor element and the described operations repeated in the exact sequence described above.

From the foregoing explanation it will be understood that each die heating interval is determined solely by the temperature to which the internal die surfaces are heated when the high frequency magnetic ileld is applied thereto. This insures that successively selected dies will be internally heated to almost exactly the same temperature, regardless of fluctuations in the voltage of the anode current supply source 38 ory fluctuations in any other variable factors which may cause variations in the density of the magnetic fields developed in the heating of different dies. This method represents a distinct departure from conventional practice, wherein the heating is effected on a timed basis utilizing conventional electronic timers for the purpose, and has distinct -advantages over such-conventional practice. Thus it makes the use of any timing device completely superfluous without requiring the use of equivalent devices. Further, by making the temperature of the die or work piece and not the elapsed time the controlling factor in determining the point at which the heating is arrested, fluctuations in the voltage of the anode current supply source 36 are rendered completely nonimportant in the operation of the apparatus to produce uniformly hardened dies. On this point it will be understood that the power which the high frequency generator i3 produces is approximately proportional to the square oi' the voltage impressed upon the anode 26a of the tube 26. Accordingly during a fixed selected time interval,

the total amount of generated heat will vary with the square of the anode voltage. In conventional practice the factor of anode voltage fluctuations is taken into account by so selecting the timing interval that the required heating is produced therein when the minimum possible voltage is applied to the anode of the oscillator tube. This oi' course results in a waste of time and heat when the available voltage actually exceeds the minimum value used as the basis for determining the heating interval.

While one embodiment of the invention has been described, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. In apparatus for heating a magnetic part having a magnetic permeability which decreases sharply as said part is heated to a predetermined quency electronic oscillator provided with a temperature, a high frequency electronic oscillator provided with a cathode-grid circuit and with an output circuit for impressing the generated high frequency magnetic field upon said part, thereby to heat said part to said predetermined temperature, the current traversing said cathodegrid circuit being sharply increased in response to said decrease in the permeability oi said part, switching means provided with an operating winding serially included in said cathode-grid circuit and operative from one setting to a different setting in response to said increase in the current -traversing said cathode-grid circuit, and means responsive to operation of said switching means to said different setting for arresting the operation of oscillator, thereby to discontinue the application of said high frequency magnetic field to said Dart.

2. In apparatus for heat treating a magnetic metal part which is movable from a heating position `to a cooling position and has a magnetic permeability which decreases sharply as said part is heated to a predetermined temperature, a high frequency electronic oscillator provided with a cathode-grid circuit and with an output circuit for impressing the generated high frequency magnetic field upon said part, thereby to heat said part to said predetermined temperature, the current traversing said cathode-grid circuit being sharply increased in response to said decrease in the permeability of said part,- switching means provided with an operating winding serially included in said cathode-grid circuit and operative from one setting to a diiIerent setting in response to said increase in the current traversing said-cathode-grid circuit, and means responsive to the operation of said switching means to said different setting for eecting movement of said part from said heating position to said cooling position.

3. In apparatus for heating a magnetic metal part which is gravity biased to move from a heating position to a cooling position and has a magnetic permeability which decreases sharply as said part is heated to a predetermined temperature, magnet means for holding said part in said heating position. a high-frequency electronic oscillator provided with a cathode-grid circuit and with an output circuit for impressing the generated high frequency magnetic field upon said part, thereby to heat said part to said predetermined temperature, the current traversing said cathode-grid circuit being sharply increased in response to said decrease in the permeability of said part, electromagnetic switching means provided with an operating winding serially included in said cathode-grid circuit and operative from one setting to a different setting in response to said increase in the current traversing said cathode-grid circuit, and means responsive to the operation of said switching means to said different setting for controlling said magnet means to release said part for movement from said heating position to said cooling position.

4. In apparatus for heating and then quenching a magnetic part having a magnetic permeability which changes as said part is heated to a predetermined temperature, a conduit for delivering a quench liquid to said part, a high frecathode-grid circuit and with an output circuit for impressing the generated high frequency magnetic field upon said part, thereby to heat said part to said predetermined temperature, the

grid circuit and operative from one setting to a different setting n response yto said increase in the current traversing said cathode-gr1d circuit. and means responsive to operation of said switching means to said different setting for discontinuing the application of said high Vfrequency magnetic ileld to said part.

HERBERT F. STORM.

REFERENCES CITED The following references are -of record in tljie file of this patent:

UNITED STATES PATENTS Number Name Date 1,779,604 Knerr Oct. 28, 1930 1,920,646 Kroger Aug. 1, 1933 1,955,352 Wallace Apr. 17, 1934 1,975,812 Wallace Oct. 9, 1934 2,059,976 Stargardter Nov. 3, 1936 2,202,759 Denneen et al May 28, 1940 2,264,301 Denneen et al. Dec. 2, 1941 2,288,033 Somes Jan. 30, 1942 2,293,851 Rogers Aug. 25, 1942 2,319,093 Somes May 11, 1943 2,324,525 Mittleman July 20, 1943 2,359,058 Somes l Sept. 26, 1944 2,395,195 Roberds Feb. 19, 1946 OTHER REFERENCES Heat Treating and Forging, pp. 89 through 92; Feb. 1941.

Electronic Industries, page 184; April 1944. 

